The cushioning function of absorbing and alleviating the shock at landing is demanded in shoe soles, in addition to the lightness in weight and the function of supporting the foot stably.
Generally, during running, a foot lands on the ground from a lateral side of a heel becomes and then inclines toward a medial side. Thus, the lateral side of the heel is subjected to large impact load of landing. Therefore, a rear foot part of the shoe sole can perform high cushioning function by deforming greatly on its lateral side. In addition, in order to restrain the inclination of the foot toward the medial side, the rear foot part of the shoe sole may be difficult to deform on its medial side, thereby performing high supporting function. Thus, it is preferred that the degree of the deformation of the shoe sole due to the shock differs between the medial side and the lateral side.
The shoe soles having an improved cushioning function are disclosed in the following patent documents.    First patent document: Japanese Patent Laid Open No. 09-285304 (abstract)    Second patent document: Japanese Patent Laid Open No. 2000-197503 (abstract)    Third patent document: Japanese Patent Laid Open No. 2002-330801 (abstract)
In the shoe soles of these documents, a member deforming due to the shock of landing is provided, and the shock of landing is absorbed by the deformation of the member. However, none of these documents discloses a point of preventing the inclination of the foot toward the medial side. And, since the deforming member is continuously provided from the medial side to the lateral side, it is difficult to adjust the difference of the degree of the deformation of the shoe sole due to the shock between the medial side and the lateral side. Thus, the shoe soles of these documents are difficult to exhibit both the shock absorption on the lateral side of the foot and the stability on the medial side of the foot.
A supported area of the deformation element divided in the rear foot part of the foot is small. Therefore, if the deformation element is made of resin foam such as EVA, a stress larger than its elastic proportional limit may be caused in the deformation element. In this case, the resin foam may undergo a great compression deformation, thereby impairing the supporting function. Permanent strain may be caused in the resin foam due to repeated stressing.
Recently, shoe soles having the repulsion function (rebound function) in addition to the above-mentioned functions have been presented. The repulsion function refers to the function of storing the impact energy at landing as deformation energy and emitting the energy of deformation when disengaging from the ground. This function is useful for improving exercise ability of a wearer.
By compressing or bending an element of the shoe sole, the deformation energy is stored in the element. However, when viscoelastic material having a small elastic proportional limit such as resin foam used for a cushioning member of the shoe sole is deformed, energy is dissipated as heat and so on. Accordingly, generally, such viscoelastic material cannot perform the repulsion function sufficiently.
The configurations of shoes having the above-mentioned repulsion function are disclosed in the following patent documents.    Fourth patent document: Japanese Patent Laid Open No. 01-274705 (abstract)    Fifth patent document: U.S. Pat. No. 6,598,320 (abstract)    Sixth patent document: U.S. Pat. No. 6,694,642 (abstract)    Seventh patent document: U.S. Pat. No. 6,568,102 (abstract)
In the shoe disclosed in Japanese Patent Laid Open No. 01-274705, a cavity is formed in the shoe sole. A reaction plate is built in this cavity. The reaction plate has upper and lower facing sides and fore and rear curved parts that connect the upper and lower facing sides. A gel cushioning member is provided in the reaction plate.
In this shoe sole, the gel cushioning member is not transversely separated nor longitudinally separated.
In the shoe sole disclosed in U.S. Pat. No. 6,694,642, hardness of the medial stabilizing pod is larger than that of the lateral stabilizing pod, but the outer sole of this shoe sole is not separated. In the shoe soles of U.S. Pat. No. 6,598,320 and U.S. Pat. No. 6,694,642, pod-like deformation elements are not arranged at three positions or more.